1. Field of the Invention
The present invention relates to a fuel pump that intakes fuel, boosts the fuel pressure, and pumps out the fuel with the boosted pressure, and more particularly to a technology for reducing frictional force acting upon an impeller of the fuel pump when the impeller rotates.
2. Description of the Related Art
A typical configuration of the conventional fuel pump will be explained below with reference to FIG. 14.
In a fuel pump 100, a motor unit 200 and a pump unit 300 are accommodated in a common housing 110. The motor unit 200 has a rotor 202. The rotor 202 has a motor shaft 204, a laminated iron core 206 fixed to the motor shaft 204, coils (not shown in the figure) wound on the laminated iron core 206, and a commutator 208 connected to end portions of each of the coils. The motor shaft 204 is supported rotatably with respect to the common housing 110 by a bearing 210 and a bearing 302 of the pump unit 300. A permanent magnet 207 is fixed inside the common housing 110 so as to surround the rotor 202. A terminal (not shown in the figure) is provided at a top cover 120 attached to the upper portion of the common housing 110. The motor unit 200 is supplied with electric power through the terminal. When the commutator 208 is supplied with electric power via a brush 212, the rotor 202 and motor shaft 204 rotate.
The pump unit 300 is accommodated in the lower portion of the common housing 110. The pump unit 300 comprises a substantially disk-shaped impeller 310, an upper casing 320, and a lower casing 330. A group of boost ports 312 is provided at an upper surface of the impeller 310 along a periphery of the impeller 310. A group of boost ports 314 is provided at a lower surface of the impeller 310 along the periphery of the impeller 310. The upper and lower casing 320,330 accommodate the impeller 310. A first boost groove 334 is formed in the lower casing 330 facing the group of boost ports 314. A second boost groove 322 is formed in the upper casing 320 facing the group of boost ports 312. When viewed along a rotational axis of the impeller 310, the first boost groove 334 and the second boost groove 322 are formed to have an almost C-like shape from an upstream end to a downstream end along the rotational direction of the impeller 310. An intake hole 332 is formed so as to be linked to the upstream end of the first boost groove 334. A discharge hole 324 is formed so as to be linked to the downstream end of the second boost groove 322. A first boost path 344 is formed by the group of boost ports 314 provided in the lower surface of the impeller 310 and the first boost groove 334 provided in the lower casing 330. A second boost path 342 is formed by the group of boost ports 312 provided in the upper surface of the impeller 310 and the second boost groove 322 provided in the upper casing 320. A central opening that engages with the motor shaft 204 is provided in the center of the impeller 310, and when the motor shaft 204 rotates, the impeller 310 also rotates.
When the impeller 310 rotates between the upper casing 320 and the lower casing 330, the fuel is sucked in from the intake hole 332 into the pump unit 300 and introduced into the boost paths 342, 344. The fuel whose pressure increases while it flows in the boost paths 342, 344 is pumped out from the fuel discharge hole 324 into the motor unit 200. The fuel that is pumped out into the motor unit 200 passes through the motor unit 200 and is pumped out to the outside from a port 122 formed in the top cover 120.
Part of the fuel under high pressure that is pumped out to the motor unit 200 flows back via a clearance around the motor shaft 204 into a space formed between the upper casing 320 and lower casing 330. This high-pressure fuel acts upon the upper surface of the impeller 310 and pushes the impeller 310 down. As a result, the impeller 310 rotates in a state of being pressed against the lower casing 330. When the impeller 310 rotates in a state in which the impeller 310 is pressed against the lower casing 330 and frictional force acts upon the impeller 310, the revolution speed of the impeller 310 decreases and pump efficiency drops.
Accordingly, the fuel pump described in International Patent Application Laid-open Publication No. WO92/011459 has a plurality of depressions 316 formed on the lower surface of the impeller 310. As shown in FIG. 15, the plurality of depressions 316 is disposed annularly and equidistantly in the circumferential direction on the inside of the group of boost ports 314 of the impeller 310. FIG. 15 is a cross-sectional view obtained when the fuel pump described in International Patent Application Laid-open Publication No. WO92/011459 is cut along the XV-XV line in FIG. 14. The edge of depression 316 on the front side in the impeller rotational direction is formed as a circular arc in the planar view thereof. The edge on the rear side has a linear shape. FIG. 16 is a cross-sectional view taken along the XVI-XVI line in FIG. 15. As shown in FIG. 16, the depression 316 is formed such that the front edge side is deeper than the rear edge side. When the impeller 310 rotates, part of the fuel located between the impeller 310 and the lower casing 330 is introduced into the depression 316, as shown by an arrow in FIG. 16. The fuel introduced into the depression 316 flows along the bottom wall surface of the depression 316 in the direction opposite to the impeller rotational direction. The fuel then flows out from the depression 316 so as to be pushed into the gap between the impeller 310 and the lower casing 330. Therefore, a pressure in the direction of separating the impeller 310 from the lower casing 330 is generated at the rear edge (boundary of the depression 316 and the gap) of the depression 316. As a result, the impeller 310 is prevented from rotating in a state of being pressed against the lower casing 330, and frictional force acting upon the impeller 310 when the impeller rotates is reduced. In the fuel pump disclosed in International Patent Application Laid-open Publication No. WO92/011459, the rear edge of the depression 316 is called a “pinch point” where the pressure is generated.